1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device including a memory cell transistor having a barrier insulating film.
2. Description of the Related Art
In a nonvolatile semiconductor memory device using a MONOS cell as a memory cell transistor, a tunnel insulating film, charge storage layer, block insulating film, and gate electrode are stacked on en element region isolated by an element isolation region. To obtain a structure in which no charge storage layer is formed between memory cell transistors in order to prevent the movement of electric charge between adjacent cells, the block insulating film is stacked on the element isolation region (see, e.g., Jae Sung Sim, “Self Aligned Trap-Shallow Trench Isolation Scheme For the Reliability of TANOS (TaN/AlO/SiN/Oxide/Si) NAND Flash Memory”, NVSMW August, 2007, Pages 110-111).
To improve the write/erase characteristics of the memory cell transistor, it is effective to use, as the block insulating film, an insulating film (high-k film) having a dielectric constant larger than that of the tunnel insulating film.
When the high-k film is used as the block insulating film, however, an impurity element produced when the high-k film is formed generates a fixed electric charge near the element region. This fluctuates the threshold voltage of the memory cell transistor, and deteriorates the characteristics of the memory cell transistor.